Hard disc drives enable users of computer systems to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive employs a stack of magnetizable discs which are rotated at a constant high speed and are provided with a plurality of nominally concentric tracks on each of the disc surfaces. Data are stored through the selective magnetization of the tracks by an array of read/write heads that are controllably positioned adjacent the tracks through the use of a closed loop digital servo system. To read previously stored data, a read/channel and disc drive interface reconstruct read signals generated as the heads detect the selective magnetization of the discs.
The tracks are defined by servo information written as a series of fields on the surfaces of the discs during disc drive manufacturing, with the fields radially aligned on the surfaces of the discs, the fields extending radially outwardly from the center of the disc in a manner similar to spokes of a wheel. User data and header fields are subsequently defined in the portions of the tracks between adjacent servo fields during a disc drive formatting operation. The user data fields (also commonly referred to as "sectors") are used to store the user data and the header fields (also commonly referred to as "headers") are used to provide the servo system with logical addressing and other information associated with the sectors, such as deallocation status.
To accommodate ever greater data storage capacities in successive generations of drives, advancements in the art have been continually implemented by disc drive manufacturers to increase data storage densities. One particularly effective advancement has been the implementation of magneto-resistive (MR) heads.
A typical MR head includes an inductive write element to write the data to a corresponding disc, with the inductive element comprising a coil wrapped about a magnetically permeable core having a write gap. Accordingly, the writing of data entails the passage of a time-varying write current through the coil which produces a magnetic field across the gap, the magnetic field selectively magnetizing the tracks.
A typical MR head further includes an MR element to subsequently read the data from the disc. The MR element comprises a material that undergoes a substantial change in electrical resistance when the element is subjected to a magnetic field of a particular orientation. Thus, the reading of data entails passing a bias current through the MR element and monitoring the voltage across the element while it is subjected to the magnetization pattern of a track.
It will be recognized that increases in data storage densities have further been achieved through the flying of the heads closer to the surfaces of the discs. However, disc drive flying heights have now been reduced to the point that the heads will occasionally contact the discs at high points on the disc (or the heads will contact contaminating particles disposed on the disc). Because of the sensitive nature of a typical MR head, such contact will usually lead to a sudden increase in temperature of the head, distorting the readback signal for several microseconds.
Such an anomalous condition is commonly referred to as a thermal asperity and will cause the readback signal to have a sudden increase in amplitude, followed by a long falling edge due to the relatively long heat dissipation time constant of the MR head. This distortion of the readback signal can impede the recovery of a significant amount of data from a track. However, the extent of such distortion is often minimized through the use of additional readback signal filtering in the read channel. Other types of anomalous conditions, such as localized media defects which prevent proper magnetization of portions of surfaces of the discs, can also detrimentally affect MR head performance; however, such can usually be minimized through the use of appropriate circuitry and compensation routines. Accordingly, MR heads have been successfully implemented into disc drives by disc drive manufacturers, but at a price of greater complexity in the construction and operation of the drives.
An example of the types of changes in operation required to utilize MR heads is the fact that the use of separate write and read elements in the same MR head generally necessitates a change in the relative positioning of the head during read and write operations. That is, as the center of the read element is usually physically offset from the center of the write element, the positioning of the head is slightly different during a read operation as compared to a write operation in order to center the respective elements over a selected track. The amount of this positional offset depends upon the construction of each particular head, as well as the radial position of the track with respect to the disc.
As a result of this positional offset, the headers used to provide addressing and other information for the associated sectors on each track are also usually offset by a corresponding amount. The headers are usually provided in pairs, with each pair comprising a write header and a read header which are read during write and read operations, respectively, on the associated sectors.
Serious operational problems thus arise when a thermal asperity (or other anomalous condition) prevents the disc drive from properly decoding the header information during a read or write operation. More particularly, as the headers serve to identify the address and status of one or more sectors, the inability to decode the header information prevents access to the sectors, which can unhappily prevent the retrieval of previously stored user data from the drive. It is to the alleviation of such header error conditions and to the facilitation of continued advancements in disc drive data storage capacities that the present invention is directed.